Diesel engines being high in torque and superior economically are the mainstream of prime movers for hydraulic excavators. Compared with gasoline engines, the diesel engines have strong robustness in combustion, and thus, it may be the case that the operation is possible even by using unprescribed fuel (kerosene, biofuel, poorly refined fuel or the like). However, if unprescribed fuel were used, assurance would not be provided in output power, fuel efficiency, exhaust gas performance and the like and at the same time, there would occur a case that the fuel constituent or the exhaust gas constituent damages various parts to finally result in a fault.
Particularly, it occurs frequently that the management in fuel is insufficient in newly rising nations and developing nations, and thus, engine faults caused by unprescribed fuel take place occasionally. Accordingly, if it is possible to detect the use of unprescribed fuel as soon as possible and to give the user a warning before the engine comes to a fault, advantages can be realized in preventing the operating rate from going down as well as in reducing the cost for maintenance.
As a technology for detecting unprescribed fuel, there is disclosed in, for example, Patent Literature 1 a technology that infers a fuel property from a detected engine operation parameter based on beforehand-calculated correlation between fuel property and the engine operation parameter. This technology determines the use of unprescribed fuel when the behavior in the engine operation parameter largely deviates from the correlation with a standard fuel property.